A low noise single photon avalanche detector with reduced dark current and a method of manufacturing the same

Abstract

The application discloses a low-noise single-photon avalanche detector with reduced dark current and a preparation method thereof. The detector comprises, from bottom to top, a substrate, an n+InP buffer layer, an i:InGaAs absorption layer, an n+InGaAsP gradient layer, an i:InP multiplication layer and a mask layer. The i:InP multiplication layer is formed with a diffusion window, the diffusion window comprises a main junction, a floating field ring and a grounding protection ring, the floating field ring is symmetrically distributed on both sides of the main junction, and the grounding protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring. Electrodes are arranged above the main junction and the grounding protection ring and below the substrate. The electric field distribution in the multiplication layer is optimized through the cooperative design of the floating field ring and the grounding protection ring, the electric field concentration at the edge is effectively inhibited, the edge breakdown is avoided, and the overall breakdown voltage of the device is determined by the main junction characteristics.

Description

Technical Field

[0001] This invention relates to the technical field of single-photon detectors, and in particular to a low-noise single-photon avalanche detector with reduced dark current and its fabrication method. Background Technology

[0002] Achieving a uniform avalanche electric field is a key challenge in the design and fabrication of InP / InGaAs SPADs. The electric field distribution within the multiplication region is often non-uniform, especially at the edges where spikes can form, with field strength even exceeding that in the central multiplication region. This non-uniformity can lead to premature edge breakdown, preventing normal avalanche multiplication. Simultaneously, the spikes at the edges induce significant tunneling effects, including interband tunneling and trap-assisted tunneling. These effects generate carriers non-photon-dependently, becoming a major cause of excessively high dark count rates (DCRs) and severely degrading the detector's signal-to-noise ratio. Therefore, optimizing the electric field distribution and suppressing edge breakdown have become key research directions for improving the performance of InP / InGaAs SPADs. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a low-noise single-photon avalanche detector with reduced dark current and its fabrication method. By employing a hybrid terminal design of a floating field ring and a grounded protection ring, the transverse electric field distribution in the multiplication region is effectively flattened, and edge spike electric fields are eliminated. This fundamentally suppresses non-photon-dependent carrier generation mechanisms such as interband tunneling and trap-assisted tunneling, thereby achieving significant suppression of dark count rate (DCR) while ensuring high photon detection efficiency.

[0004] To achieve the above objectives, the technical solution provided by this invention is as follows: a low-noise single-photon avalanche detector with reduced dark current, comprising at least one epitaxial structure unit, wherein the epitaxial structure unit comprises, from bottom to top, a semi-insulating InP substrate, an n+ InP buffer layer, an i: InGaAs absorption layer, an n+ InGaAsP graded layer, an i: InP multiplication layer, and a mask layer; the mask layer has multiple apertures for performing a single Zn diffusion, and the i: InP multiplication layer has diffusion windows formed through the apertures, wherein the diffusion windows include a main junction, a floating field ring, and a ground protection ring, wherein the floating field ring is symmetrically distributed on both sides of the main junction, and the ground protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring; the mask layer has contact holes located above the main junction and the ground protection ring.

[0005] Furthermore, a first metal layer serving as an anode is provided inside the contact hole, and a second metal layer serving as a cathode is provided below the semi-insulating InP substrate.

[0006] Furthermore, the grounding protection ring is connected to the second metal layer through the first metal layer.

[0007] Furthermore, for a single epitaxial structure unit, there is only one main junction, two floating field rings are symmetrically distributed on both sides of the main junction and form a spacing ratio of 0.5-2 with the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field ring and maintain a preset gap with the floating field ring.

[0008] Furthermore, for a single epitaxial structure unit, there is only one main junction, four floating field rings are symmetrically distributed on both sides of the main junction and form a spacing ratio of 0.5-2 with the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field ring and maintain a preset gap with the floating field ring.

[0009] Furthermore, the thickness of the n+ InP buffer layer is 500 nm; the thickness of the i: InGaAs absorber layer is 1.5 μm to 3 μm; the thickness of the n+ InGaAsP gradient layer is 100 to 200 nm; and the thickness of the i: InP multiplication layer is 2.5 to 3.5 μm.

[0010] A method for fabricating a low-noise single-photon avalanche detector with reduced dark current as described above includes the following steps:

[0011] S1, from bottom to top, a semi-insulating InP substrate, an n+ InP buffer layer, an i: InGaAs absorber layer, an n+InGaAsP gradient layer and an i: InP multiplication layer are arranged sequentially.

[0012] S2. Diffusion mask layer in i:InP multiplication layer, and then use photolithography to etch multiple apertures in mask layer to perform Zn diffusion;

[0013] S3. In Zn diffusion, i: InP multiplication layer forms diffusion windows through multiple apertures. The diffusion windows include the main junction, floating field ring and ground protection ring. The floating field ring is symmetrically distributed on both sides of the main junction, and the ground protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring.

[0014] S4. Using photolithography, contact holes are opened in the mask layer. The contact holes are only located at the center of the main junction and on the protective ring. No contact holes are opened above the floating field ring, which is used to force the floating field ring to be in an electrically floating state.

[0015] S5. A first metal layer as the anode is deposited at the location of the contact hole, and a second metal layer as the cathode is deposited under the semi-insulating InP substrate. The P-type electrode in the grounding protection ring is led out and connected to the second metal layer through the first metal layer to form at least one epitaxial structure unit of a low-noise single-photon avalanche detector.

[0016] Furthermore, step S1 includes:

[0017] The thickness of the n+ InP buffer layer is 500 nm; the thickness of the i: InGaAs absorber layer is 1.5 μm to 3 μm; the thickness of the n+ InGaAsP gradient layer is 100 to 200 nm; and the thickness of the i: InP multiplication layer is 2.5 to 3.5 μm.

[0018] Furthermore, step S3 includes:

[0019] Zn diffusion is performed in the i:InP multiplication layer using pores with a diameter of 15~30 μm to form a main junction. The main junction is a PN junction with high surface concentration and shallow junction depth, which serves as the active region for avalanche multiplication. The doping concentration is 0.8 - 2 × 10¹⁸ cm⁻³.

[0020] The floating field rings are symmetrically distributed around the main junction. They are formed by Zn diffusion in the i:InP multiplication layer using narrow ring apertures with a diameter of 3-6 μm. The concentration is diluted due to the significant lateral diffusion effect. The floating field rings are floating regions with medium effective doping concentration and medium junction depth. The doping concentration is 2 - 6 × 10¹⁷ cm⁻³. The spacing ratio between the floating field rings and the main junction is 0.5-2, which ensures full coupling in the depletion region to achieve a smooth electric field transition, while avoiding the doping profiles from penetrating each other.

[0021] The grounding protection rings are symmetrically distributed around the floating field rings. The grounding protection rings are formed by Zn diffusion in the i:InP multiplication layer using the narrowest ring aperture of 2~3µm in diameter. The lateral diffusion dilution effect is used to form the grounding protection rings, which are the regions with the lowest effective doping concentration and the deepest junction depth in the entire field. They are used to connect to a fixed potential and serve as the final node for terminating the electric field. The distance between the grounding protection rings and the floating field rings is maintained at 0.5~3µm, and the doping concentration is 0.5 -2 × 10¹⁷ cm⁻³.

[0022] Furthermore, step S3 includes:

[0023] For a single epitaxial structure unit, there is only one main junction, two or four floating field rings are symmetrically distributed on both sides of the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field rings.

[0024] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0025] 1. This invention optimizes the electric field distribution within the i:InP multiplication layer through the synergistic design of a floating protection ring and a ground protection ring. The floating protection ring creates a higher breakdown voltage step around the main junction, forcing breakdown to occur at the center of the main junction. Meanwhile, the ground protection ring clamps the potential at the chip edge to a low level and collects surface leakage current, guiding it to the ground electrode. This design effectively suppresses electric field concentration at the edges, thereby preventing premature edge breakdown and ensuring that the overall breakdown voltage of the device is determined by the characteristics of the main junction.

[0026] 2. In this invention, the main junction, floating field ring, and grounding protection ring undergo only one Zn diffusion process, saving costs and time. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the extensional structural unit.

[0028] Figure 2 This is a schematic diagram of the arrangement of diffusion windows on a single epitaxial structural unit.

[0029] Figure 3 This is a schematic diagram of the electric field distribution of the i: InP multiplication layer. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments.

[0031] Example 1

[0032] See Figure 1 As shown, the low-noise single-photon avalanche detector with reduced dark current provided in this embodiment includes at least one epitaxial structure unit. The epitaxial structure unit includes, from bottom to top, a semi-insulating InP substrate 1, an n+InP buffer layer 2, an i:InGaAs absorption layer 3, an n+InGaAsP gradient layer 4, an i:InP multiplication layer 5, and a mask layer 6.

[0033] The mask layer 6 has multiple apertures for primary Zn diffusion; i: diffusion windows are formed in the InP multiplication layer 5 through these apertures, see [link / reference]. Figure 2As shown, the diffusion window includes a main junction 7, a floating field ring 8, and a ground protection ring 9. In each epitaxial structure unit, there is only one main junction 7. There are two floating field rings 8 symmetrically distributed on both sides of the main junction 7, with a spacing ratio of 0.5~2 between the floating field rings 8 and the main junction 7. There are two ground protection rings 9 symmetrically distributed on both sides of the floating field rings 8, maintaining a gap of 0.5~3µm between them. The mask layer 6 has contact holes located above the main junction 7 and the ground protection rings 9. A first metal layer 10 serving as the anode is provided inside the contact holes. A second metal layer 11 serving as the cathode is provided below the semi-insulating InP substrate 1. The ground protection ring 9 is connected to the second metal layer 11 through the first metal layer 10. The P-type electrode in the ground protection ring 9 is connected to the second metal layer 11 through the first metal layer 10 via a lead 12.

[0034] Finally, through the coordinated design of the floating field ring 8 and the grounding protection ring 9, the electric field distribution within the i:InP multiplication layer 5 was optimized. (See [link to relevant documentation]). Figure 3 As shown, the floating field ring 8 forms a higher breakdown voltage step around the main junction 7, forcing breakdown to occur at the center of the main junction 7. Meanwhile, the ground protection ring 9 clamps the potential at the chip edge to a low level and collects surface leakage current, guiding it to the ground electrode. This design effectively suppresses electric field concentration at the edges, thereby preventing premature edge breakdown and ensuring that the overall breakdown voltage of the device is determined by the characteristics of the main junction.

[0035] Example 2

[0036] This embodiment provides another low-noise single-photon avalanche detector with reduced dark current. Unlike embodiment 1, in each epitaxial structure unit, there is only one main junction 7, four floating field rings 8 are symmetrically distributed on both sides of the main junction 7, the spacing ratio between the floating field rings 8 and the main junction 7 is 0.5~2, and two grounding protection rings 9 are symmetrically distributed on both sides of the floating field rings 8 and maintain a gap of 0.5~3um with the floating field rings 8.

[0037] Example 3

[0038] This embodiment provides a method for fabricating a low-noise single-photon avalanche detector with reduced dark current as described in Embodiment 1 or 2, including the following steps:

[0039] S1. A semi-insulating InP substrate, an n+ InP buffer layer, an i: InGaAs absorber layer, an n+ InGaAsP gradient layer, and an i: InP multiplication layer are arranged sequentially from bottom to top; wherein, the thickness of the n+ InP buffer layer is 500 nm; the thickness of the i: InGaAs absorber layer is 1.5 μm to 3 μm; the thickness of the n+ InGaAsP gradient layer is 100 to 200 nm; and the thickness of the i: InP multiplication layer is 2.5 to 3.5 μm.

[0040] S2. Diffusion mask layer in i:InP multiplication layer, and then use photolithography to etch multiple apertures in mask layer to perform Zn diffusion;

[0041] S3. In Zn diffusion, i: InP multiplication layer forms diffusion windows through multiple apertures. The diffusion windows include the main junction, floating field ring and ground protection ring. The floating field ring is symmetrically distributed on both sides of the main junction, and the ground protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring.

[0042] Zn diffusion is performed in the i:InP multiplication layer using pores with a diameter of 15-30 μm to form a main junction. There is only one main junction per unit cell. The main junction is a PN junction with high surface concentration and shallow junction depth, serving as the active region for avalanche multiplication. The doping concentration is 0.8-2 × 10⁻². 18 cm⁻³;

[0043] The floating field rings are symmetrically distributed around the main junction, with 2 or 4 rings per unit cell. They are formed by Zn diffusion in the i:InP multiplication layer using narrow ring apertures of 3–6 μm in diameter. Utilizing the significant lateral diffusion effect leading to concentration dilution, the floating field rings represent floating regions with moderate effective doping concentration and moderate junction depth. The doping concentration is 2 - 6 × 10⁻⁶. 17 cm⁻³, the spacing ratio between the floating field ring and the main junction is 0.5~2, to ensure full coupling in the depletion region so as to achieve a smooth transition of the electric field, while avoiding the interconnection of doped profiles;

[0044] The grounding protection rings are symmetrically distributed around the floating field ring, with two rings per unit. They are formed by Zn diffusion in the i:InP multiplication layer using the narrowest ring aperture of 2-3 μm in diameter. The lateral diffusion dilution effect is utilized. The grounding protection rings represent the region with the lowest effective doping concentration and the deepest junction depth in the entire field, and are used to connect to a fixed potential, serving as the final node terminating the electric field. The distance between the grounding protection rings and the floating field rings is maintained at 0.5-3 μm, and the doping concentration is 0.5 - 2 × 10⁻². 17 cm⁻³.

[0045] S4. Using photolithography, contact holes are created in the mask layer. The contact holes are only located at the center of the main junction and on the guard ring. No contact holes are created above the floating field ring to force the floating field ring into an electrically floating state. Only in this way can it obtain an adaptive floating potential that varies with the bias voltage through the capacitive coupling effect, thereby dynamically and efficiently smoothing the peak electric field at the edge of the main junction and increasing the breakdown voltage to the required level.

[0046] S5. A first metal layer as the anode is deposited at the location of the contact hole, and a second metal layer as the cathode is deposited under the semi-insulating InP substrate. The P-type electrode in the grounding protection ring is led out and connected to the second metal layer through the first metal layer to form at least one epitaxial structure unit of a low-noise single-photon avalanche detector.

[0047] Finally, through the coordinated design of the floating field ring 8 and the grounding protection ring 9, the electric field distribution within the i:InP multiplication layer 5 was optimized. (See [link to relevant documentation]). Figure 3 As shown, the floating field ring 8 forms a higher breakdown voltage step around the main junction 7, forcing breakdown to occur at the center of the main junction 7. Meanwhile, the ground protection ring 9 clamps the potential at the chip edge to a low level and collects surface leakage current, guiding it to the ground electrode. This design effectively suppresses electric field concentration at the edges, thereby preventing premature edge breakdown and ensuring that the overall breakdown voltage of the device is determined by the characteristics of the main junction.

[0048] The above-described embodiments are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Therefore, any changes made in accordance with the shape and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A low-noise single-photon avalanche detector with reduced dark current, comprising at least one epitaxial structure unit, characterized in that: The epitaxial structure unit includes, from bottom to top, a semi-insulating InP substrate, an n+ InP buffer layer, an i:InGaAs absorber layer, an n+ InGaAsP gradient layer, an i:InP multiplication layer, and a mask layer. The mask layer has multiple apertures for primary Zn diffusion. The i:InP multiplication layer has diffusion windows formed through the apertures. The diffusion windows include a main junction, a floating field ring, and a ground protection ring. The floating field ring is symmetrically distributed on both sides of the main junction, and the ground protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring. The mask layer has contact holes located above the main junction and the ground protection ring.

2. The low-noise single-photon avalanche detector with reduced dark current according to claim 1, characterized in that: The contact hole contains a first metal layer serving as an anode, and a second metal layer serving as a cathode is disposed below the semi-insulating InP substrate.

3. The low-noise single-photon avalanche detector with reduced dark current according to claim 2, characterized in that: The grounding protection ring is connected to the second metal layer through the first metal layer.

4. The low-noise single-photon avalanche detector with reduced dark current according to claim 1, characterized in that: For a single epitaxial structure unit, there is only one main junction, two floating field rings are symmetrically distributed on both sides of the main junction and form a spacing ratio of 0.5-2 with the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field ring and maintain a preset gap with the floating field ring.

5. A low-noise single-photon avalanche detector with reduced dark current according to claim 1, characterized in that: For a single epitaxial structure unit, there is only one main junction, four floating field rings are symmetrically distributed on both sides of the main junction and form a spacing ratio of 0.5-2 with the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field ring and maintain a preset gap with the floating field ring.

6. The low-noise single-photon avalanche detector with reduced dark current according to claim 1, characterized in that: The thickness of the n+ InP buffer layer is 500 nm; the thickness of the i: InGaAs absorber layer is 1.5 μm to 3 μm; the thickness of the n+ InGaAsP gradient layer is 100 to 200 nm; and the thickness of the i: InP multiplication layer is 2.5 to 3.5 μm.

7. A method for fabricating a low-noise single-photon avalanche detector with reduced dark current according to any one of claims 1-6, characterized in that, Includes the following steps: S1, from bottom to top, a semi-insulating InP substrate, an n+ InP buffer layer, an i: InGaAs absorber layer, an n+InGaAsP gradient layer and an i: InP multiplication layer are arranged sequentially. S2. Diffusion mask layer in i:InP multiplication layer, and then use photolithography to etch multiple apertures in mask layer to perform Zn diffusion; S3. In Zn diffusion, i: InP multiplication layer forms diffusion windows through multiple apertures. The diffusion windows include the main junction, floating field ring and ground protection ring. The floating field ring is symmetrically distributed on both sides of the main junction, and the ground protection ring is symmetrically distributed on both sides of the floating field ring and maintains a preset gap with the floating field ring. S4. Using photolithography, contact holes are opened in the mask layer. The contact holes are only located at the center of the main junction and on the protective ring. No contact holes are opened above the floating field ring, which is used to force the floating field ring to be in an electrically floating state. S5. A first metal layer as the anode is deposited at the location of the contact hole, and a second metal layer as the cathode is deposited under the semi-insulating InP substrate. The P-type electrode in the grounding protection ring is led out and connected to the second metal layer through the first metal layer to form at least one epitaxial structure unit of a low-noise single-photon avalanche detector.

8. The method for fabricating a low-noise single-photon avalanche detector with reduced dark current according to claim 7, characterized in that, Step S1 includes: The thickness of the n+ InP buffer layer is 500 nm; the thickness of the i: InGaAs absorber layer is 1.5 μm to 3 μm; the thickness of the n+ InGaAsP gradient layer is 100 to 200 nm; and the thickness of the i: InP multiplication layer is 2.5 to 3.5 μm.

9. The method for fabricating a low-noise single-photon avalanche detector with reduced dark current according to claim 7, characterized in that, Step S3 includes: Zn diffusion is performed in the i:InP multiplication layer using pores with a diameter of 15-30 μm to form a main junction. The main junction is a PN junction with high surface concentration and shallow junction depth, serving as the active region for avalanche multiplication. The doping concentration is 0.8 - 2 × 10⁻⁶. 18 cm⁻³; The floating field rings are symmetrically distributed around the main junction. They are formed by Zn diffusion in the i:InP multiplication layer using narrow ring apertures of 3–6 μm in diameter. Utilizing the significant lateral diffusion effect leading to concentration dilution, the floating field rings represent floating regions with moderate effective doping concentration and moderate junction depth. The doping concentration is 2–6 × 10⁻⁶. 17 cm⁻³, the spacing ratio between the floating field ring and the main junction is 0.5~2, to ensure full coupling in the depletion region so as to achieve a smooth transition of the electric field, while avoiding the interconnection of doped profiles; The grounding protection rings are symmetrically distributed around the floating field rings. They are formed by Zn diffusion in the i:InP multiplication layer using the narrowest ring aperture of 2-3 μm in diameter. Through lateral diffusion dilution, the grounding protection rings represent the region with the lowest effective doping concentration and the deepest junction depth in the entire field. They are used to connect to a fixed potential, serving as the final node terminating the electric field. The distance between the grounding protection rings and the floating field rings is maintained at 0.5-3 μm, and the doping concentration is 0.5 - 2 × 10⁻² μm. 17 cm⁻³.

10. The method for fabricating a low-noise single-photon avalanche detector with reduced dark current according to claim 9, characterized in that, Step S3 includes: For a single epitaxial structure unit, there is only one main junction, two or four floating field rings are symmetrically distributed on both sides of the main junction, and two grounding protection rings are symmetrically distributed on both sides of the floating field rings.

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